Particulates comprising silica and alumina, and methods of utilizing these particulates in subterranean applications

ABSTRACT

The present invention involves reduced specific gravity particulates and their use in subterranean applications such as production enhancement and completion. One embodiment of the present invention provides particulates comprising silica and an aluminum oxide and at least one void and having a specific gravity of less than about 2.2, a particle size of 8 U.S. Mesh or smaller, and a substantially spherical shape. Other embodiments of the present invention provide for using such particulates in subterranean operations such as fracturing and gravel packing.

FIELD OF THE INVENTION

The present invention relates to improved particulates and methods ofusing such particulates in subterranean applications. More particularly,the present invention relates to reduced specific gravity particulatesand their use in subterranean applications such as productionenhancement and completion.

DESCRIPTION OF THE PRIOR ART

Servicing fluids comprising particulates are used in a variety ofoperations and treatments performed in oil and gas wells. Suchoperations and treatments include, but are not limited to, productionstimulation operations such as fracturing and well completion operationssuch as gravel packing.

An example of a production stimulation operation using a servicing fluidhaving particles suspended therein is hydraulic fracturing. That is, atype of servicing fluid, referred to in the art as a fracturing fluid,is pumped through a well bore into a subterranean zone to be stimulatedat a rate and pressure such that fractures are formed and extended intothe subterranean zone. The fracture or fractures may be horizontal orvertical, with the latter usually predominating, and with the tendencytoward vertical fractures increasing with the depth of the formationbeing fractured. The fracturing fluid is generally a gel, emulsion, orfoam that may comprise a particulate material often referred to asproppant. The proppant is deposited in the fracture and functions, interalia, to hold the fracture open while maintaining conductive channelsthrough which such produced fluids can flow upon completion of thefracturing treatment and release of the attendant hydraulic pressure.

An example of a well completion operation using a treating fluidcontaining particulates is gravel packing. Gravel packing treatments areused, inter alia, to reduce the migration of unconsolidated formationparticulates into the wellbore. In gravel packing operations, particlesreferred to in the art as gravel are carried to a well bore in asubterranean producing zone by a hydrocarbon or water carrier fluid.That is, the particulates are suspended in a carrier fluid, which may beviscosified, and the carrier fluid is pumped into a well bore in whichthe gravel pack is to be placed. As the particulates are placed in thezone, the carrier fluid leaks off into the subterranean zone and/or isreturned to the surface. The resultant gravel pack acts as a filter toseparate formation solids from produced fluids while permitting theproduced fluids to flow into and through the well bore. While screenlessgravel packing operations are becoming more common, traditional gravelpack operations involve placing a gravel pack screen in the well boreand packing the surrounding annulus between the screen and the well borewith gravel sized to prevent the passage of formation particulatesthrough the pack with produced fluids. The gravel pack screen isgenerally a filter assembly used to support and retain the gravel placedduring the gravel pack operation. A wide range of sizes and screenconfigurations are available to suit the characteristics of a well bore,the production fluid, and any particulates in the subterraneanformation. When installing the gravel pack, the gravel is carried to theformation in the form of a slurry by mixing the gravel with aviscosified carrier fluid. Once the gravel is placed in the wellbore,the viscosity of the carrier fluid is reduced and it is returned to thesurface. Such gravel packs are used to stabilize the formation whilecausing minimal impairment to well productivity. The gravel, inter alia,acts to prevent the particulates from occluding the screen or migratingwith the produced fluids, and the screen, inter alia, acts to preventthe gravel from entering the well bore.

In some situations the processes of hydraulic fracturing and gravelpacking are combined into a single treatment to provide a stimulatedproduction and an annular gravel pack to prevent formation sandproduction. Such treatments are often referred to as “frac pack”operations. In some cases the treatments are completed with a gravelpack screen assembly in place with the hydraulic fracturing treatmentbeing pumped through the annular space between the casing and screen. Inthis situation the hydraulic fracturing treatment ends in a screen outcondition creating an annular gravel pack between the screen and casing.This allows both the hydraulic fracturing treatment and gravel pack tobe placed in a single operation. In other cases the fracturing treatmentmay be performed prior to installing the screen and placing a gravelpack.

Fracturing fluids, gravel packing carrier fluids and frac pack fluidsgenerally must be highly viscous to be able to suspend particulates. Toachieve a high viscosity, viscosifiers often are added to such fluids.Such viscosifiers are expensive. Moreover, as a fracture or a gravelpack is created a portion of the liquid contained in the fluid may leakoff into the formation and/or may create an undesirable filter cakecomprising deposited viscosifier on the walls of the fracture, wellbore, or the formation.

Filter cakes are sometimes desirable to aid in preventing drilling andother servicing fluids from being lost in the formation and to preventsolids from entering the porosities of the producing formation. However,just as a filter cake may block the loss of fluids into the formation,the same filter cake may block the production of fluids from theformation. Thus, the presence of a filter cake on a producing zone isgenerally undesirable when a subterranean formation is returned toproduction. Moreover, residue of viscosifiers used in subterraneanapplications often remains on the particulates transported in theviscosified fluid and may reduce the conductivity of packs made fromsuch particulates.

Also, as more wells are being drilled in deep water and in hightemperature zones, gravel packing in long open horizontal well bores isbecoming more prevalent. Completion operations in these wells generallyinvolve the use of reduced-specific gravity particulates that areresistant to degradation in the presence of hostile conditions such ashigh temperatures and subterranean treatment chemicals. In order toprevent damage to these producing zones by gravel packing operations,the treating fluid carrying the particles should generally exhibit arelatively low viscosity. Similarly, fracture stimulation treatmentscarried out in deep, high temperature wells require similarreduced-specific gravity, spherical particles in lower viscosity fluids.

Traditional high-strength particulates used in subterranean applicationsoften exhibit too high of a specific gravity to be suspended in suchlower viscosity fluids. While low specific gravity particulates, such aswalnut hulls are well known in the art, generally they are not able towithstand significant closure stresses over time at elevatedsubterranean temperatures. Similarly, a variety of light-weightparticles formed of thermoplastic materials including polyolefins,polystyrene divinylbenzene, polyfluorocarbons, polyethers etherketonesand polyamide imides are commercially available. However, when theseparticles are exposed to temperatures above about 150° F., they softenand deform, and are not suitable in all well bores.

SUMMARY OF THE INVENTION

The present invention relates to improved particulates and methods ofusing such particulates in subterranean applications. More particularly,the present invention relates to reduced specific gravity particulatesand their use in subterranean applications such as productionenhancement and completion.

One embodiment of the present invention provides particulates comprisingsilica and an aluminum oxide, at least one void, and have a specificgravity of less than about 2.2, a particle size of 8 U.S. mesh orsmaller, and a substantially spherical shape.

Another embodiment of the present invention provides particulatescomprising at least about 30% silica, less than about 25% aluminumoxides, and a plurality of internal voids.

Still another embodiment of the present invention provides a method offracturing a subterranean formation comprising the steps of: providing afirst fluid; providing a second fluid comprising particulates whereinthe particulates comprise silica and an aluminum oxide, at least onevoid, and have a specific gravity of less than about 2.2, a particlesize of 8 U.S. mesh or smaller, and a substantially spherical shape;placing a first fluid into the subterranean formation at a pressuresufficient to create or enhance at least one fracture therein; placing asecond fluid into the subterranean formation and fracture; reducing theviscosity of the first fluid; reducing the viscosity of the second fluidso as to deposit the particulates into the fracture; substantiallyremoving the first fluid and second fluid from the fracture whileleaving at least a portion of the particulates in the fracture. Thefirst fluid may be the same as or different than the second fluid.

Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the description of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to improved particulates and methods ofusing such particulates in subterranean applications. More particularly,the present invention relates to reduced specific gravity particulatesand their use in subterranean applications such as productionenhancement and completion.

The reduced-specific gravity particulates of the present inventioncomprise combustion products of carbonaceous materials such as oil,wood, garbage, sewage, hydrocarbons, coal, and the like. Typically, suchcombustion products suitable for use in the present invention comprisefrom about 30 to about 70 percent by weight silica and from about 5 toabout 25 percent by weight aluminum oxides, with the majority of thebalance being oxides of calcium, magnesium, potassium, sodium, iron, andtitanium. Generally, the combustion products also comprise materialsthat will form gas when exposed to sintering temperatures.

In some embodiments of the present invention, the combustion productsmay comprise “fly ash.” Fly ash, as referred to herein, refers to afinely divided residue resulting from the combustion of carbonaceousmaterial, such as ground or powdered coal, and generally carried bygenerated flue gases. One preferred fly ash is ASTM class F fly ash,having a Blaine fineness of about 10,585 square centimeters per gram andcommercially available from Halliburton Energy Services, Inc., ofHouston, Tex., under the trade designation “POZMIX®.” In otherembodiments of the present invention, the combustion product maycomprise “bottom ash.” Bottom ash, as referred to herein, refers to afinely divided residue resulting from the combustion of carbonaceousmaterial and generally accumulating on the floor of an incinerator.

The particulates of the present invention have low specific gravities,preferably below about 2.2. To create particulates having a suitably lowspecific gravity, it is desirable to control the respective percentagesof metal oxides in the final particulate. For example, Table 1 belowshows the pure compound specific gravities for a variety of commonoxides:

TABLE 1 Specific Gravity of Metal Oxides Fe₂O₃ 5.2 TiO₂ 4.2 Al₂O₃ 4.0MgO 3.6 CaO 3.3 Al₂O₃.SiO₂ 3.2 CaO.Al₂O₃ 3.0 CaO.Al₂O₃.2SiO₂ 2.8CaO.SiO₂ 2.5 SiO₂ 2.3

As shown in Table 1, aluminum oxides, such as Al₂O₃, are relativelyheavy and, as such, the relative degree that these are present in theparticulates of the present invention should be considered in light ofthe desired specific gravity of the particulates. The prior art,however, teaches that aluminum oxides are necessary to impart sufficientcrush-resistance to a particulate so that it is useful in subterraneanapplications. Nevertheless, it has been determined that suitablecrush-resistance may be obtained even while the concentration ofaluminum oxides in the particulates is greatly reduced. In fact, in someembodiments of the present invention, the percentage by weight ofaluminum oxides can be reduced to about 25%, about 10%, or even to zeroand yet still produce a product with a crush strength suitable for usein most subterranean applications. By way of example, in one embodimentof the present invention, a particulate may comprise about 60% silica,about 20% alumina oxides, and about 5% calcium oxides.

The particulates of the present invention generally exhibit crushstrengths such that they can withstand pressures of at least about 1,000psi without undesirable performance. One way to determine particulatecrush strength is the place a known weight of particulates into acylinder, apply a known pressure to the cylinder over a one minute timeperiod, hold the pressure for two minutes, and then, once the pressureis released, to determine the weight of the fines produced by thepressure. If the weight of the fines is less than about 20% of the totalweight of particulates placed in the cylinder than the particulates canbe said to have withstood the pressure applied.

In some preferred embodiments of the present invention, the particulatesare capable of withstanding a pressure of at least about 2,500 psi. Byway of example, a particulate comprising about 53% by weight SiO₂ andabout 26% by weight Al₂O₃ has been shown to withstand pressures of about5,000 psi without substantially degrading.

The particulates of the present invention may be made by a processcomprising pelletizing and sintering the compositions of the presentinvention. One embodiment comprises forming a composition comprisingcombustion products and a binder; pelletizing the mixture into discreteparticulates; and sintering the discrete particulates at temperaturesabove about 1000° C. to produce reduced-density particulates.Optionally, the discrete particulates may be dried or roasted attemperature between about 65° C. to about 150° C. prior to sintering theparticulates. Such drying acts, inter alia, to reduce the particulate'swater content.

Binders used in sintering, such as clay, are well known in the art andit is within the ability of one skilled in the art to choose a binderfor use in the sintering step of the methods of the present invention.Also known in the art is the fact that a fluid, such as water, iscommonly used with the binder in a pelletizing process.

By way of example, in some embodiments of the present invention, thecombustion products comprise from about 50% to about 80% by weight ofthe mixture to be pelletized and the binder comprises from about 0.1% toabout 5% by weight of the mixture to be pelletized. Where a fluid suchas water is used along with the binder in a pelletizing process, thefluid may comprise from about 10% to about 25% by weight of the mixtureto be pelletized.

When such combustion products are pelletized and sintered, they produceparticulates that are substantially spherical and that exhibit specificgravities of below about 2.2. The term “spherical” is used in thisinvention will designate pellets having an average ratio of minimumdiameter to maximum diameter of about 0.7 or greater. The size of theparticulates of the present invention is generally about 8 U.S. mesh orsmaller. Having such a particle size allows the particulates to beuseful in sand control operations such as gravel packing and productionenhancing operations such as fracturing. One skilled in the art with thebenefit of this disclosure will recognize the appropriate size for agiven application.

Processes capable of producing spherical pellets are well known in theart. One such method that may be used to produce the particulates of thepresent invention involves placing combustion products, binder, and afluid such as water into a high intensity mixer. One such suitable highintensity mixer is the RV02 High Intensity Mixer available from EirichMachines, Inc. of Gurnee, Ill. As the mixture exits the high intensitymixer, it may then be poured onto a slanted, rotating table. The size ofthe particulate may be influenced, inter alia, by affecting the speed ofthe table's rotation and the angle of the table's slant.

The specific gravity and crush strength of the particulates of thepresent invention may be influenced, in part, by adjusting, inter alia,the percentage of materials in the combustion products that will formgas when sintered; the residence time; the rate of heating in thesintering process; and, the final sintering temperature. For example, asthe percentage of materials in the combustion products that will formgas when heated increases, the porosity of the sintered particulate willincrease, thus decreasing the particulate's specific gravity andcrush-resistance. Moreover, as the residence time in the sinteringprocess and the final sintering temperature increase, the specificgravity of the particulate may increase as porosities within theparticulate decrease.

As the percentage of voids in the particulate increases, its specificgravity will decrease and its crush strength will decrease. Bycontrolling the level of voids in the particulate it is possible totailor a proppant for the specific gravity and crush strength needed ina particular application. When the particulate is sintered, thematerials in the combustion products that will form gas when heatedleave behind voids and pore spaces in the particulate. Thus, it followsthat the level of materials in the combustion products that will formgas when sintered in the pelletized particulate prior to sintering isrelated to the final level of porosity of the particulate. Thus, as therate of heating in the sintering process increases, the specific gravityof the particulate may decrease as more porosities are created by therapid formation of gas in the particulate.

The particulates of the present invention may be heated by any meansknown in the art. The heating may be batch or continuous. In onecontinuous heating method, a rotary kiln may be used. A rotary kiln maybe a refractory lined cylinder that is fired either directly ofindirectly. In a directly fired rotary kiln, heat is provided at one endof the rotating cylinder and the feed of particulates to be heated isprovided at the opposite end. Manipulatable kiln parameters include therevolution rate of the cylinder, the tilt angle of the cylinder, thefinal kiln temperature, and the temperature gradient along the cylinder.One skilled in the art with the benefit of this disclosure will be ableto determine the kiln parameters needed to produce a desiredparticulate.

If a higher porosity particulate is desired, the particulate may beflash-roasted prior to sintering. Flash roasting acts, inter alia, torapidly drive gases and residual water from the pelletized particulateand to create fissures within the particulate. Flash roasting hereinrefers to rapidly heating the particulates. If the heating occurs at ahigh enough rate, the materials that will form gas when heated willquickly exit the particulate. Such a rapid exit yields a porousstructure wherein the at least some pores communicate with the exterior.Such pores may be thought of as “intercommunicating pores” because theyconnect a point inside the particulate with the particulate's surfaceand the environment surrounding the particulate. Flash roasting may beperformed in a standard fluidized bed reactor. Generally, in such areactor heated gases are fed from below a charge of particulates suchthat the particulates are lifted and mixed at a high rate while beingheated. One flash roaster suitable for use in the present invention is aTorbed® reactor commerically available from Torftech Limited, FerndaleCourt, West End Road Mortimer, Reading RG7 3SY, Berkshire, UnitedKingdom.

Another method of increasing the final level of porosity of theparticulate is to include a clay material in the feed mixture ofcombustion products. Clays inherently contain water molecules that willrelease when the particulate is flash-roasted or sintered.

Particulates having intercommunicating pores may be used to delivertreatment chemicals to a subterranean formation. A treatment chemicalmay be introduced into the pores of the particulate and then theparticulate may be delivered to the subterranean formation. Thetreatment chemical may then exit the pores via diffusion. Chemicalssuitable for delivery on porous media include but are not limited to gelbreakers, oxidizers, enzymes, hydrolyzable esters, scale inhibitors,biocides, corrosion inhibitors, paraffin inhibitors, and substantiallyany other chemical that is soluble in the formation fluids under theenvironmental conditions of the formation within which it is placed. Theparticulates of the present invention may be impregnated with any of theabove chemicals by any means known in the art.

In some embodiments of the present invention, at least one of the voidscreated in the particulate is an internal void. That is, the void isentirely enclosed inside the particulate and does not communicate withthe particulate's surface. Where it desirable to minimizeintercommunicating pores and create, instead, internal voids, one methodof forming such voids is to expose the particulate to higher sinteringtemperatures for a brief period of time such that the external surfaceof the particle vitrifies, thus forming a skin on at least a portion ofthe outer layer of the particulate such that voids communicating withthe particulates may be sealed off.

Many subterranean treatments require that particulates be suspended in atreatment fluid and carried into the subterranean formation. Generally,the treatment fluid must exhibit a viscosity high enough to suspend theparticulates. The reduced specific gravity of the particulates of thepresent invention allows for the use of lower viscosity servicingfluids. In one embodiment of the present invention, a treatment fluidcomprising a hydrocarbon or water carrier fluid comprisingreduced-specific gravity particulates of the present invention suspendedtherein is pumped into a subterranean formation and at least a portionof the particulates are deposited therein.

One embodiment of a method of the present invention provides an improvedmethod of treating a subterranean formation using a treatment fluidcomprising a hydrocarbon or water carrier fluid and reduced-specificgravity particles of the present invention suspended therein. In thetreatment methods of the present invention, a treatment fluid comprisinga hydrocarbon or water carrier fluid comprising reduced-specific gravityparticles of the present invention suspended therein is placed into asubterranean formation and then the carrier fluid's viscosity is reducedso as to deposit at least a portion of the reduced-specific gravityparticles in a desired location in the subterranean formation.

Still another method of the present invention provides an improvedmethod of hydraulic fracturing using reduced specific gravityparticulates of the present invention. In some hydraulic fracturingmethods of present invention, a first fluid is placed in thesubterranean formation at a rate and pressure sufficient to form atleast one fracture in the subterranean formation. Next, a second fluidcomprising particulates of the present invention is placed into thefractures. Next, the viscosity of the first fluid is reduced, interalia, to facilitate its removal from the subterranean formation and theviscosity of the second fluid is reduced, inter alia, to deposit thesuspended particulates in the fracture and, inter alia, to facilitatethe removal of the second fluid from the subterranean formation.Finally, the reduced viscosity first fluid and second fluid aresubstantially removed from the fracture and the subterranean formation.

Another method of the present invention provides an improved method ofgravel packing a delivery fluid comprising a hydrocarbon or watercarrier fluid comprising reduced-specific gravity particles of thepresent invention suspended therein. In some of the gravel packingmethods of the present invention, a gravel pack composition comprising acarrier fluid having reduced-specific gravity particles of the presentinvention suspended therein is placed in a region of a well bore suchthat at least a portion of the gravel particles form a gravel packsubstantially adjacent to the well bore. Then the viscosity of thedelivery fluid remaining in the well bore may be reduced so that it maythen be removed from the subterranean formation.

Therefore, the present invention is well adapted to carry out theobjects and attain the ends and advantages mentioned as well as thosethat are inherent therein. While numerous changes may be made by thoseskilled in the art, such changes are encompassed within the spirit andscope of this invention as defined by the appended claims.

1. Particulates comprising silica, an aluminum oxide in an amount ofabout 0.1 percent to about 25 percent by weight, and at least one void,the particulates having a specific gravity of less than about 2.2, aparticle size of 8 U.S. Mesh or smaller, and a substantially sphericalshape, wherein the at least one void communicates between a point insideof the particulate, and a surface of the particulate and an environmentsurrounding the particulate.
 2. The particulates of claim 1 comprisingfrom about 30 percent to about 70 percent by weight silica.
 3. Theparticulates of claim 1 further comprising calcium oxides.
 4. Theparticulates of claim 1 wherein the silica and aluminum oxide comprisecombustion products of carbonaceous materials.
 5. The particulates ofclaim 1 comprising a particle size of 25 U.S. mesh or smaller.
 6. Theparticulates of claim 1 wherein the particulate is capable ofwithstanding a closure stress of at least about 2,500 psi.
 7. Theparticulates of claim 1 wherein at least a portion of the outer layer isvitrified.
 8. The particulates of claim 1 further comprising a treatmentchemical for treating a subterranean formation.
 9. The particulates ofclaim 8 wherein the treatment chemical comprises at least one chemicalselected from the group consisting of: a gel breaker, an oxidizer, anenzyme, a hydrolysable ester, a scale inhibitor, a biocide, a corrosioninhibitor, and a paraffin inhibitor.
 10. Particulates comprising atleast about 30% silica, aluminum oxides in an amount of about 0.1percent to about 25 percent by weight, a plurality of internal voids,and a substantially spherical shape, wherein at least one of theplurality of internal voids communicates between a point inside of theparticulate, and a surface of the particulate and an environmentsurrounding the particulate.
 11. The particulates of claim 10 comprisingat least about 40 percent by weight silica.
 12. The particulates ofclaim 10 comprising less than 20% by weight aluminum oxides.
 13. Theparticulates of claim 10 further comprising calcium oxides.
 14. Theparticulates of claim 10 wherein the silica and aluminum oxides comprisecombustion products of carbonaceous materials.
 15. The particulates ofclaim 10 comprising a particle size of 25 U.S. mesh or smaller.
 16. Theparticulates of claim 10 wherein the particulate is capable ofwithstanding a closure stress of at least about 2,500 psi.
 17. Theparticulates of claim 10 wherein at least a portion of the outer layeris vitrified.
 18. The particulates of claim 10 further comprising atreatment chemical for treating a subterranean formation.
 19. Theparticulates of claim 18 wherein the treatment chemical comprises atleast one chemical selected from the group consisting of: a gel breaker,an oxidizer, an enzyme, a hydrolysable ester, a scale inhibitor, abiocide, a corrosion inhibitor, and a paraffin inhibitor.